Apparatus and method for removing metal from the surface of a metal object



E. R; HOLMAN 2,884,728 APPARATUS AND METHOD FOR REMOVING METAL FROM May 5, 1959 THE SURFACE OF A METAL OBJECT .2 SheetsSheet 1 Filed Aug. 16, 1954 .2 o w.v v. 3 1' Z 6 7 6 v m m a 0m w m M i w I 14 A w 6 m 5 i 2 QMM mm INVENTOR HOLMFIN HERTER EMMETTE BY E. HOLMAN 2 Sheets-Sheet 2 INVENTOR HT TOR NE y.

APPARATUS AND METHOD FOR REMOVING METAL FROM THE SURFACE OF A METAL OBJECT May 5, 1959 Filed Aug. 16, 1954 w 2 2 z 2 1 12m W mdw w e 0 T f 5 J! O K H v alliilllrill mw 6 1 v r 5 0 5 4 w 8 Mm 6 Z 7 2 1 8 M 8 P 9 United States Patent APPARATUS AND METHOD FOR REMOVING FROM THE SURFACE OF A METAL B Emmette 1R. Holman, Pasadena, Calif., assignor to Turco Products, Inc., Los Angeles, Calif., a corporation of California Application August 16, 1954, Serial No. 449,994

8 Claims. (Cl. 41-9) This invention relates to the etching or the removal of metal from the surface of a metal object, e.=g., one composed of iron, nickel, magnesium, zinc, aluminum or their alloys. More particularly, the invention is concerned with novel apparatus and procedures for this purpose, especially for the treatment of aluminum and its alloys.

It is known to etch acid soluble metals such as magnesium, zinc and their alloys with an acid solution such as aqueous nitric acid. It is also known to etch alkali soluble metals such as aluminum and its alloys with a solution having a solvent action on the aluminum or alloy surface, such as a hot aqueous alkali solution, e.g., one containing sodium hydroxide. One method of carrying out the etching process employing acids is to immerse the metal part to be etched in the etching bath, While another is to splash the solution contained in a bath against the work, the surface of which is to be etched or dissolved away. The prior art method for etching alkali soluble metals such as aluminum or its alloys is by immersion. However, the use of the conventional immersion or splash methods for etching metal parts often produces non-uniform etching, is difficult to control, and is time consuming.

Moreover, where aluminum and its alloys are to be etched, conventional procedures have proven unsatisfactory because of the formation of a precipitate of aluminum hydroxide and/ or alumina in the solvent or etching bath, particularly when the concentration of alkali in the solution is reduced due to the reaction of the alkali with the metal. The formation of such precipitate not only interferes with the action of the etching solution on the work but tends to form a scale on the etching tank or equipment which becomes most difiicult to remove.

If the tanks are heated by coils, this precipitate in the form of a hard scale, deposits on the coils and also on the surface of the etching tanks; additionally, by depositing on the walls, the scale reduces the volumetric capacity of the tank and if allowed to continue will fill the tank. In order to prevent or reduce the formation of hard adherent scale, it has been proposed to add to the alkaline etching solution, agents which are said to prevent the reversion of the aluminate to form the aluminum hydroxide and alumina, and to cause the precipitate which is formed to be granular and nonadherent. These agents include, for example, gluconic acid, saccharic acid, mucic acid, or salts thereof, sorbito], and the like. non-adherent precipitates is citric acid or salts thereof, employed in an amount equal to more than 2 parts by weight of citric acid or an equivalent amount of sodium salt of the acid, to about 96.8 parts by weight of free caustic soda, as described and claimed in application, Ser. No. 445,197 filed July 22, 1954, by Kenneth W. Newman, now Patent No. 2,739,883.

However, even in the above cases, the granular precipi- A useful agent to produce flocculent tate tends to fill the tank and must be removed periodically. Also, the strength of the solution gradually decreases and the etching solution must be revitalized by addition of fresh amounts of alkali.

One object of this invention is to devise novel apparatus for readily dissolving or removing metal in a controllable manner from the surface of a metal object, e.g., iron, nickel, aluminum, magnesium, zinc, and the like or their alloys.

Another object is to provide apparatus for the controlled etching of a metal part.

Another object is to design novel apparatus particularly adapted for the uniform, controlled etching or removal of metal from the surface of a work piece composed of aluminum or its alloys.

A still further object of the invention is the provision of novel apparatus for the efficient controlled removal of metal from the surface of an aluminum or aluminum alloy part using a solvent or etching bath, including means for maintaining the bath substantially free of solids or precipitate which becomes suspended in the solvent bath.

Yet another object is to provide novel apparatus for the efiicient etching or removal of metal from the surface of an aluminum or aluminum alloy work piece by controllably splashing or directing an alkali solution contained in a bath against said work piece, and including means for the rapid removal of the solids formed in the etching reaction, from said bath, and for maintaining the strength of said solution.

A still further object is the provision of procedure for the efl'icient removal of metal from the surface of a metal work piece such as aluminum or an aluminum alloy using a suitable solvent solution, while maintaining the solution at proper strength and substantially free of solids or precipitate which becomes suspended in the solvent solution.

Other objects and advantages will become apparent from the following description of the invention:

According to the invention metal solvent or etching solution contained in a bath is repeatedly splashed upwardly to form streams or sheets of liquid which by means of especially designed paddles are made to impinge on a work piece suitably mounted for this purpose. The metal part to be' etched is coated on selected portions of the surface with a protective masking coating to inhibit or prevent attack by the etching agent while adjacent portions are available for such attack. The etched work piece may be used in the decorative arts or in structures or machine parts. The etching solution is lifted from the body of the etching bath by each of the paddles of the invention device and directed against each work piece or part to be etched, preferably in the form of streams having an essentially constant direction and uniform velocity. This causes a uniform rate of attack of the metal of the work piece, which can be controlled by varying the velocity or amount of liquid in the impinging streams, e.g. by varying the nature of the streams say from fine to heavy drops of liquid, or by changing the direction of the streams through proper adjustment of the paddles, or by varying the rate of immersion and withdrawal of the paddles from the solution. The invention apparatus includes means for readily making the above changes and adjustments to obtain the desired etching results.

This procedure is carried out preferably in an especially designed tank in the presence of air. The surface of the metal is thus corroded by the solution in the desired or unmasked areas of the work piece, and aeration of the solution while being splashed against the work piece produces a scouring effect against the surface thereof. Thus, particularly where the work piece being processed is aluminum or an alloy thereof, such scouring action aids in the removal of so-called smut often formed on the surface of the work piece during aluminum etching, and also of any adherent aluminum compounds such as the oxide or hydroxide. Smut appears on surfaces of aluminum alloys, particularly when aluminum 'is' alloyed with elements, such as for example, copper,

iron, silicon and carbon, etc. Such aluminum alloys when etched with caustic soda, produce a film which is black and highly adherent to the surface. These smuts are insoluble in water and most acids. They are soluble in HNO mixtures of HNO and H 80 and in mixtures of chromic and sulfuric acids. Such acids are extremely hazardous to use industrially.

The blast of the etchant solution against the surface 'of the Work piece also functions to sweep away the blanket of hydrogen foam, as well as the aforementioned smut and the tenacious aluminum oxide, all of which have a masking efiect which interferes or blocks the etching reaction. The solvent solution, after impinging on the work piece, drops back into the bath, carrying with it the aforementioned solids and gaseous foam.

A portion of the liquid mixture from the bath and containing solids or precipitate such as aluminum hy- 'droxide and/or alumina formed in the etching reaction and suspended in the bath, is removed to a second zone maintained at a lower temperature, wherein the solids are settled out of the mixture and removed, and the remaining clear liquor is withdrawn. Such liquor may .be recirculated directly to thebath, but is first prefer- ,ably conducted to a third zone in which make-up liquid (water, alkali or other components of the bath) can be added to the solution, if desired, to replenish or revitalize the main etching or solvent bath. The resulting solution can then be heated to the proper temperature and recirculated to the etching bath in the first zone. The process of removal of solids and recirculation of liquor to the bath may be carried out either intermittently or continuously, preferably the latter.

Apparatus found particularly advantageous for carrying out the above procedure includes an enclosed tank adapted to receive a body of liquid (the solvent bath) in the lower portion thereof, and including a fluid inlet and outlet. A rotatable shaft is mounted longitudinally in the tank between the top and bottom thereof, the shaft having mounted near each end for rotation therewith a series of radial coplanar spokes in the form of a wheel, the spokes of the oppositely positioned wheels being substantially in radial alignment. An adjustable paddle is positioned between each pair of aligned spokes on the wheels adjacent the opposite ends of the shaft. Rods are fixedly and non-rotatably mounted at opposite ends of the shaft normal thereto, the rods being longer than .said spokes. Supported at the ends of the rods and above the lower portion of the tank are a pair of aligned arcuately shaped supports having mounted therebetween and parallel to the shaft, a series of adjustably positioned rods. These rods carry retainers or clamps for mounting the work pieces to be treated by the solvent or etching solution. Means may also be provided to raise and lower the level of the liquid in the tank for a reason hereinafter pointed out.

The outlet at the bottom of the etching tank is connected to a settling vessel having means therein to facilitate rapid settling of the suspended solids in the liquid mixture conveyed from the main tank, and means to permit withdrawal of the resulting clear liquor. Such liquor is then conducted to a make-up tank having agitating means, and wherein make-up liquid can be added to the clear liquor from the settling vessel. The resulting solution is then pumped through a heater back to the inlet of the main etching tank for addition to the etching bath therein.

' The invention will be more clearly understood by reference to the following description of a preferred 4 embodiment taken in connection with the accompanying drawings wherein:

Fig. l is an essentially schematic assembly view of the apparatus of the invention, shown partly in section and partly broken away for clarity;

Fig. 2 is an irregular section in elevation of the etching tank shown in Fig. 1, certain parts being omitted for clarity;

Fig. 3 is a section taken on line 33 of Fig. 2;

Fig. 4 shows an elevational view of a detail of the device of Fig. 2 taken on line 44 of Fig. 2; and

Fig. 5 is a broken plan view taken of the structure of Fig. 4.

Numeral 10 represents a tank in which metal work pieces are etched or subjected to the solvent action of a. body of solution 12 maintained in the lower portion of the tank according to the invention. Where the work piece is an acid soluble metal such as magnesium, zinc or their alloys, the solvent solution can be of suitable acid composition such as aqueous nitric acid. According to the preferred embodiment of the invention for etching or removing metal from an aluminum or aluminum alloy material, which is alkali soluble, a hot aqueous solution is employed containing an alkali such as sodium or potassium hydroxide, trisodium phosphate, soda ash or the like, or mixtures thereof. Preferably a sodium hydroxide solution is used. Temperature of the solution in the etching tank is generally maintained in a range say from F. to about 210 F. during treatment.

It is preferred in the instances where the part being treated is an acid soluble metal such as magnesium, zinc or their alloys, that an insoluble saturated aliphatic acid of from 5 to 26 carbon atoms or a petroleum sulfonate be incorporated in the acid bath in order to produce a deeper etch while minimizing undercutting, according'to US. Patents Nos. 2,640,763 and 2,640,767. Examples of such materials are valeric, lauric, palmitic and stearic acids, and naphthenic sulfonates such as the sulfonates corresponding to mahogany soaps. Where the part being etched is aluminum or an alloy thereof, other immiscible liquid additives to facilitate etching may be incorporated in the etching bath. The additive forms a separate oily layer of liquid 13 which floats on top of the main body of solvent or etching solution 12 in tank 10.

It is preferred in practice of the invention to incorporate -a sludge modifying agent into the alkali etching bath, for example, citric acid or its sodium, potassium or ammonium salt, as disclosed in the above mentioned Newman application.

Tank 10 has a liquid inlet 14 in the lower side of the tank and an outlet 16 in the bottom thereof. The tank is formed of a lower rectangular portion 18 having a sloping bottom 20, and a semi-cylindrical upper cover portion 22 closed at opposite ends, the lower ends of which are provided with flanges 24 mating with flanges 26 at the upper ends of the lower portion 18 of the tank, for clamping the cover portion 22 by means of nut and bolt assemblies 28 to the lower portion of tank 10.

Mounted in bearings 30 and 31 on opposite end walls 32 and 34 of the tank 10 about midway of the side walls 36 thereof is a shaft 38 which extends horizontally and longitudinally of the tank 10. Shaft 38 is parallel to walls 36 and is positioned at about the juncture of cover 22 and lower portion 18 of the tank, and hence, as seen in Fig. 1, is located along the axis of the semi-cylindrical cover portion 22. A hub 40 is fixed to shaft 38 near one end thereof and a number of equally spaced radial spokes 44 are connected to hub 40 for rotation with the shaft. Near the opposite end of shaft 38 is another hub 46 which is connected to a series of radial spokes 48 similar to spokes 44, there being the same number of spokes 48 as 44, and spokes 48 being of the same length as spokes 44 and in radial alignment therewith.

Spokes 44 and 48 have apertures 50 at the ends thereof for receiving the pins 52 located at opposite ends of flat paddles or blades 54, each of which is adapted to be connected across the ends of a pair of radially aligned spokes 44 and 48. Each paddle 54 is secured against longitudinal movement by nuts 56 placed on pins 52 at opposite ends of the paddle. Paddles 54 are positioned parallel to shaft 38, and the paddles can be adjusted to change the plane or pitch thereof by loosening nuts 56 and rotating the paddles on their axes to the desired position. Collars 42 are positioned between the respective hubs 40 and 46 and the adjacent bearings 30 and 31 to prevent axial movement of shaft 38. Collars 42 are adjustably mounted for rotation with the shaft 38 by means of set screws 43.

Mounted on bearings 30 are a series of radial rods 58 having a length greater than spokes 44 and 48. These rods extend only a little more than half way about shaft 38, as seen in Fig. 2, and are essentially located in the cover portion 22 of the tank 10. The same number of like rods 60 in radial alignment with rods 58 extend outwardly from hearing 31 at the opposite end of shaft 38. The ends of rods 58 are connected to a U-shaped support member 62, and the ends of rods 60 are connected to a similar aligned U-shaped support member 64. Each of the members 62 and 64 have oppositely aligned apertures 66 therein. Members 62 and 64 have approximately the same contour as the cover portion 22, these support members being spaced from the paddles 54 to permit free rotation of the wheels 68 and 70, and the paddles supported thereon.

Received in the respective pairs of opposite aligned apertures 66 of support members 62 and 64, are rods 72 which extend longitudinally of tank parallel to paddles 54 and shaft 38. Along each of rods 72 are suspended brackets 74, shown as three in number in Fig. 3. The lower end of brackets 74 (see Fig. 2) have a threaded aperture 76 therein adapted to receive a screw 78 having a lock nut 80 thereon, the opposite end of the screw being adapted to be forced into contact with the sides of a work piece 84 to maintain it in a fixed position, as seen in Figs. 2 and 5. If desired, clamps, e.g. of an adjustable type, may be connected to the screws 78 for holding the work pieces. Rods 72 may be adjusted in the various apertures 76 to accommodate work pieces of different sizes.

' One end of shaft 38 extends through wall 34 of the tank 10 externally thereof, a pulley 86 being keyed to the outer end of shaft 38 and driven by a belt 88 for rotation of the shaft and wheels 68 and 70 thereon. A packing gland 82 is positioned on shaft 38 adjacent hearing 31 to prevent leakage. Standards 87 are provided in tank 10 adjacent the walls 32 and 34 thereof to support shaft 38 and its associated structure. The standards are connected to the lower portion 18 of the tank 10 by means of the bolt and nut assemblies 92, which also serve to connect the vertical flange 90 connected to the bottom 20 of the tank, to the walls 32, 34 and 36 thereof to form the lower portion 18 of the tank 18.

It will be seen from Fig. 2 that the respective paddles 54 dip into the body of solution composed of liquids 12 and 13 in the tank as shaft 38 and the wheels 68 and 70 thereon are rotated, and the paddles direct or splash the solution against the work pieces 84 as the paddles move upwardly out of the solution. The solution which impinges on the work reacts therewith to etch the same, and the resulting liquid carrying solids and foam drains off the work pieces and back into the body of solution in the lower portion 18 of the tank.

The design of the paddles and the position of the work pieces with respect to the paddles is such that a uniform spray of a mixture of liquids 12 and 13 impinges against each of the work pieces. That is, the amount of liquid and the velocity of each successive series of streams leaving each of the paddles and striking a given work piece are substantially the same, as is the direction of such streams. In other words, these parameters remain substantially constant during the entire treatment. Since a uniform amount of etching liquid is directed substantially in a constant direction against the respective work pieces, a uniform quantity of fresh solution is constantly in contact with the metal work pieces, bringing about a controlled rate of attack of the metal. The impinging liquid sweeps across the surface of the metal carrying with it deposited material such as aluminum hydroxide (where aluminum or its alloys is being treated), and the resulting reaction fluid, including smut and foam, then drains back into the main body of liquid in the tank. This procedure applies also to treatment of acid soluble metals, except that no precipitate is formed in the etching reaction.

Thus, referring to Fig. 2, as wheel 68 is rotated counterclockwise, each paddle 54 directs a stream of liquids 12 and 13 in the form of coarse droplets against the first work pieces 84 mounted to the right as viewed in Fig. 2, and; as the respective paddles continue to rotate, the streams directed by the paddles against the successive work pieces are composed of droplets of progressively smaller size. Hence, the streams striking the work pieces 84 mounted at the top of the tank are composed of droplets of intermediate size, while the streams impinging on the work pieces mounted to the extreme left are made up of the finest droplets. It is noted that the droplets in the streams emitted from the paddles as they emerge from the bath are coarsest, and the droplets in the streams emitted from the paddles just prior to immersion thereof are of the finest size. The droplets detach themselves from the leading edge of the paddles in a \direction roughly tangential to the peripheral arc of rotation. Some of the droplets fall back against the following paddles, and these are in turn picked up and propelled peripherally against subsequent work pieces.

If it is desired to change the direction or amount of liquid in the streams of liquid impinging against the metal work pieces 84, the pitch of the blades or pad dles 54 can be adjusted to the desired position. For this purpose, means e.g. in the form of adjustable clamps (not shown) may also be provided for changing the angle of the work pieces to receive a spray which is at any desired angle to the impinging streams. The velocity of the liquid streams impelled by the paddles can be changed by varying the speed of rotation of wheels 68 and 70. The position of the work pieces can be arranged along the arcuate supports 62 and 64 so that those work pieces which it is desired to treat with a finer, e.g., mist-like spray of etchant are located to the left in the tank, as seen in Fig. 2, while those to be treated by coarser streams of etching liquid are positioned to the right in the tank.

The relative proportions of fluid 12 and fluid 13 which are lifted out of the tank and directed against the work pieces by each paddle depends on the extent of !dip of the paddles in the body of solution, the greater the immer sion of the paddles the larger will be the amount of alkali solution to additive solution 13 in the liquid streams leaving the paddles. Also, the degree of dip of the paddles and the velocity of rotation of the paddle wheels determine the trajectory of the various size droplets in the streams produced by the respective paddles. Thus, for example, a short dip paddle rotating comparatively slowly, will throw a stream of coarse droplets having a relatively fiat trajectory, which stream will impinge on the lowest work pieces to the right adjacent the point of departure of the paddle from the solution (viewing Fig. 2 and assuming counterclockwise rotation of the paddle wheels), while the streams of finer droplets will have a steeper trajectory so that these streams will impinge on successive work pieces as the paddles rotate. If the paddles are deeply immersed in the body of solution and the paddle wheel is rotating comparatively rapidly, the trajectory of the various streams of coarse and fine droplets will be greater respectively than in the first case; that is, the streams containing the coarsest particles may strike say, the second work piece to the right in Fig. 2

7 following removal of the paddle from solution with comparatively little fluid from such paddle striking the first work piece to the right, while the streams of smaller droplets are distributed against the work pieces mounted near the top and to the left in the upper portion of the tank, with the streams of smallest droplets being directed against the last or lowest work piece to the left as viewed in Fig. 2.

To remove from the body of etching solution in tank 10 suspended solids in the form of aluminum hydroxide and/or alumina, resulting from the reaction of the alkali with the aluminium or aluminum alloy work pieces 84, as previously described, a portion of the liquid mixture is continuously removed by means of the tank outlet 16 and is passed via valve 95 by gravity through a pipe 96 to a settling vessel 98. Vessel 98 has a scraper 100 positioned adjacent the bottom 102 of the vessel, the scraper being rotatably supported on a central shaft 104, which is rotated by a motor 106 suitably mounted on the top 108 of vessel 98. Pipe 96 discharges into a cylindrical bafie 110 suitably mounted axially within vessel 98, cansing flow of discharged fluid and solids downwardly into the vessel. The fluid then rises from the bottom of the vessel 98 through the annular space 109 between the baflie 110 and the side wall 111 of the vessel, and overflows into a trough 112 mounted about the upper periph cry of vessel 98. The path of fluid flow is indicated by the arrows in Fig. 1. In this manner the suspended solids in the liquid mixture entering vessel 98 are deposited at the bottom of the vessel, and substantially clear liquor flows into the trough 112.

Rotation of scraper 100 maintains the solids in a thick suspension at the bottom of vessel 98, a portion of such solids being continuously discharged through the outlet 114 of vessel 98 into a hopper or conveyor 116 having a screw feed 118 therein mounted on a shaft 120, the end of which passes through a packing gland 122 at the adjacent end of the hopper, the shaft 120 being driven by a motor 123. The screw conveyor 118 discharges the solid material through openings 124 at the outer end of the hopper.

The clear solution overflowing into trough 112 is conveyed by a pipe 126 through valve 127 to a make-up tank 128. Here the solution is agitated by an agitator 130 driven by a motor 132, while make-up liquid may be added through pipe 134. The make-up liquid can be simply water or additional alkali solution 12 or additive liquid 13, or any combination thereof, to replenish that lost in the main tank 10.

The solution in tank 128 is discharged via valve 135 into pipe 136 and to the intake of pump 138 which pumps the liquid first through a heater 140 to bring the solution up to the desired temperature and then circulates the heated liquid through pipe 142 connected to the inlet 14 of the tank 10, and into the main body of solvent or etching solution in such tank.

The liquid level in tank 10 can be adjusted by manipulation of valves 95, 127 and 135 For this purpose a sight glass 144 is positioned in the side of tank 10. In this manner, the amount of dip of the paddles 54 into the body of solution can be changed as desired to produce streams having the desired angles or trajectories with respect to the work pieces. If desired, other means may be employed for changing the position of the shaft and paddles with respect to the surface of the body of liquid in tank 10.

The alkali concentrations employed range from 0.1 to 10 normal and if sludge modifiers are used, they may be employed in amounts sufiicient to produce a non-adherent sludge or precipitate as for example, 0.5% to 10% by weight of the alkali. When employing citric acid it is used in a ratio of more than 1 mol of citrate ion to 230 mols of OH ion. This is conveniently obtained by dissolving a mixture of at least 2 parts by weight of citric acid to 98 parts by weight of granular caustic soda. 1

may conveniently use from 2.7 parts by weight of citric acid to 97.3 to parts by weight of granular soda, dissolved in water. Instead of NaOH I may use any of the other equivalent alkalis referred to above adjusting the concentration to give the desired OH ion concentration. I may use the sodium, potassium or ammonium salt in amounts equivalent to the citric acid as stated above, adjusting hydroxyl ion concentration to give the above ratios.

As an example, work pieces of 24 ST aluminum alloy are mounted in tank 10 as described above. A solution composed of 7% by weight of a mixture of NaOH and citric acid (4 parts by weight of granular citric acid and 96 parts by weight of granular NaOH) is dissolved in water in the tank and circulated through heater to a temperature of degrees F. The paddles are rotated to give the spray distribution desired for the degree of etch desired in each work piece. Sludge is precipitated, settled in tank 10, and is withdrawn together with some solution from the tank and passed into settling tank 98 where it settles and is withdrawn by means of the conveyor 116. The overflow passes to tank 128 and fresh water, alkali and citric acid, if necessary, are added to adjust the concentration of the solution to that originally used, and the resulting solution reheated in heater 140 to 180 F. and passed to tank 10.

While the above described apparatus and procedure for removal of precipitated solids in the etching bath is preferred and produces best results when employed in combination with the splash technique and apparatus therefor described above, such procedure for solids removal may be employed to advantage in connection with other etching techniques such as immersion.

Instead-of supporting paddles or blades 54 on a pair of wheels 68 and 70, I may employ a single wheel mounted approximately centrally along shaft 38, the paddles being adjustably mounted centrally at the ends of the spokes of said wheel. Also, if desired, I may employ more than two wheels for adjustably supporting the paddles.

While the etching bath has been shown in the above described embodiment to consist preferably of the main etching solution 12, an immiscible additive 13, and a sludge or precipitate modifying agent, if desired, one or both of the latter may be omitted.

The process of the invention may be entirely continuous. The materials of construction of the apparatus of my invention are those which are resistant to attack by acids where an acid etching bath is employed, or by alkali where an alkali etching bath, e.g., sodium hydroxide, is utilized.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A device for the removal of metal from metal work pieces which comprises a case adapted to contain a body of liquid in the lower portion thereof, a rotatable shaft. mounted in said case, said shaft being positioned parallel to a longitudinal axis of said case, a plurality of first spokes radiating from said shaft adjacent the end thereof, a plurality of second spokes radiating from said shaft adjacent the opposite end thereof, said first spokes and said second spokes forming a pair of parallel wheels each in a plane substantially normal to said axis, the respective spokes of said wheels being in alignment, a substantially flat adjustable paddle mounted between the ends of each pair of aligned spokes on said wheels, said paddles being spaced fro-m said case, and a plurality of retaining members mounted in the upper portion of said case about a portion of the periphery of said wheels, said retaining members being spaced from said paddles and being adapted to hold said metal work pieces in position.

2. A device for the removal of metal from metal work pieces which comprises a case adapted to contain a body of liquid in the lower portion thereof, a rotatable shaft mounted in said case longitudinally thereof, a plurality of radial coplanar first spokes mounted on said shaft normal to and adjacent one end thereof, a plurality of radial coplanar second spokes mounted on said shafit normal to and adjacent the other end thereof, said spokes being mounted for rotation with said shaft, the respective first and second spokes being in alignment with each other, a paddle mounted between each pair of aligned spokes adjacent the outer ends thereof, a plurality of first radial rods fixedly and nonrotatably mounted on said shaft adjacent said first spokes, a plurality of second radial rods fixedly and nonrotatably mounted on said shaft adjacent said second spokes, said rods being longer than said spokes, a first support connected adjacent the ends of said first rods, a second support connected adjacent the ends of said second rods, and a plurality of adjustably positionable clamps for said metal work pieces, said clamps being mounted on said supports.

3. A device for the removal of metal from the surface of metal work pieces, which comprises an enclosed tank adapted to receive a body of liquid in the lower portion thereof, a fluid inlet to and a fluid outlet from said tank, a rotatable shaft mounted longitudinally in said tank, between the top and bottom thereof, a plurality of radial coplanar first spokes mounted on said shaft normal to and adjacent the end thereof, a plurality of radial coplanar second spokes mounted on said shaft normal to and adjacent the other end thereof, said spokes being mounted for rotation with said shaft, the respective first and second spokes being in alignment with each other, an adjustably positionable paddle mounted between each pair of aligned spokes at the outer ends thereof, a plurality of first radial rods fixedly and nonrotatably mounted on said shaft between said first spokes and said tank, a plurality of second radial rods fixedly and nonrotatably mounted on said shaft between said second spokes and said tank, said rods being longer than said spokes, a first arcuate support connected adjacent the ends of said first rods, a second arcuate support connected adjacent the ends of said second rods and in alignment with said first arcuate support, a plurality of adjustable parallel third rods mounted on said supports, and a plurality of clamps for said metal work pieces, said clamps being disposed on said third rods.

4. A device for the removal of metal from the surface of a metal Work piece which comprises in combination a first tank, an inlet to said tank, an outlet from said tank, a rotatable shaft mounted in said tank, a paddle mounted on said shaft for rotation therewith, means for supporting said metal work piece in said tank adjacent to but spaced from said paddle, a second tank, a first pipe connecting said outlet with said second tank, a rotatable scraper positioned adjacent the bottom of said second tank, means for rotating said scraper, a 'bafile adjacent the inlet to said second tank for directing incoming fluid downward in said second tank, a trough mounted about the periphery of said second tank in the upper portion thereof, means connected to the bottom of said second tank for withdrawing solid material therefrom, a third make-up tank, a second pipe connecting said trough with said third tank, a make-up liquid inlet to said third tank, an agitator in said third tank, means for operating said agitator, an outlet from said third tank, a third pipe connecting said last named outlet with the inlet to said first tank, a pump in said third pipe and a heater in said third pipe.

5. A process of etching an aluminum object, which comprises reacting said aluminum object with an aqueous solution of an alkaline etching agent in an etching zone to dissolve aluminum from said object, forming in said solution an alumina precipitate, separating said precipitate from said solution in a zone other than the etching zone, whereby the formation of said alumina precipitate returns to said etching solution the alkaline etching agent originally reacted with said object to dissolve the aluminum forming said alumina, by withdrawing the solution containing the alkaline etching agent from said other zone, replenishing the consumed constituents in said withdrawn solution, and returning the replenished solution to the etching zone.

6. A process of etching an aluminum object as set forth in claim 5, wherein the temperature of the solution in said etching zone is maintained at a value higher than that in said other zone.

7. A process of etching an aluminum object as set forth in claim 6, wherein said withdrawn solution is heated prior to its return to said etching zone.

8. A process of etching an aluminum object as set forth in claim 5, wherein the etching solution is caused to circulate continuously from said etching zone, to said other zone, and back to said etching zone.

References Cited in the file of this patent UNITED STATES PATENTS 82,652 Spain Sept. 29, 1868 362,971 Jones May 17, 1887 1,166,378 Levy Dec. 28, 1915 1,146,071 Hoffman July 13, 1915 1,716,270 Holmstrom June 4, 1929 1,908,487 Powers May 9, 1933 1,947,391 Guenst Feb. 13, 1934 2,396,685 Coggins Mar. 19, 1946 2,405,105 Kennedy July 30, 1946 2,668,130 Martin Feb. 2, 1954 FOREIGN PATENTS 15,410 Austria Mar. 10, 1904 583,929 France Nov. 10. 1924 

